Electric machines performances are characterized by a certain number of factors, the importance of which depends on the type of targeted applications. A major sizing element is the motor constant, also called Km, and expressed in torque units per Watt square root, i.e. reflecting the torque that the motor can generate for a given power consumption in the coils. Another factor which may be important is the torque without current, which must often be reduced as much as possible for various reasons (noise, loss, mechanical wear, geared motor reversibility . . . ).
Within the scope of this patent, optimizing the motor constant for given axial overall dimensions and diameter will be our concern, while striving to have a motor with a minimum torque without current in motors having a “radial” topology, i.e. having a plane flow and radially extending teeth. The axial overall dimension will be defined here as the height of the stator plus the height of the coils heads above the stator. More generally, the capacity of industrializing such a motor is obviously a background factor to be taken into account. Eventually, we will also be preferentially interested by structures with balanced radial forces (making it possible to solve problems such as noise or wearing of rotor guide elements). The criteria mentioned above generally lead to compromises in the definition of structures that do not always enable to obtain an optimized structure.
For low to medium power motors, topologies with 6 or 12 stator teeth are currently found, with the 12 teeth topology making it possible to obtain the best performance in terms of Km. The stator topologies having 12 teeth with equal peak divergences make it possible to eliminate, in association with magnetized rotors having a number of pairs of poles equal to 5+2R (with R being an integer) and not a multiple of 3, the harmonics of the torque without current up to the 12th row (excluded). The first appearing and majority harmonic thus has a period 12 times smaller than the electric period of the motor. As the amplitude of the harmonics decreases with their row, these motors are particularly valuable for applications requiring a minimum torque in the absence of current.
The U.S. Pat. No. 7,595,577 is known in the prior art which relates to a motor having 12 equal teeth extended at their ends on the stator. This solution is a classic way to minimize the torque without current in demanding applications in the field. As a matter of fact, as the torque without current is linked to the preferred positions that the magnet rotor can take due to the gaps between the stator teeth, it seems wise to minimize these spaces, or even in some cases to eliminate these by contacting the teeth in order to reduce the twelfth harmonic.
However, this type of solution has several drawbacks. First, the closer position of, or the contact between the teeth, induces leakage fluxes which affect the motor performances, even though the contact areas (commonly called necks) are so arranged as to be magnetically saturable. Moreover, these topologies require either a direct winding in the slots, or executing the stator in several subsets, which may be annoying in terms of industrial production. Eventually, the space available for the copper of the windings is not optimal.
The German patent application DE102009000681 is also known, which describes an electric machine comprising: a rotor having 14 poles, with the rotor being so arranged as to be rotatable about a central axis, and a stator having 12 stator teeth which protrude in a radial direction relative to the central axis in the direction of the rotor characterized in that each stator tooth is provided with a stator coil, wherein two adjacent stator coils are each time connected in series to each other to form a pair of stator coils, with one pair of stator coils being associated with a phase being each time connected to a dedicated neutral point, so to form a star point circuit. For this type of embodiment of the prior art, the straight tooth synchronous machine provides for a tooth width as wide as that of the notches, generally a tooth width of about 15°. The figures of patent DE102009000681 do not make it possible to clearly understand the peak divergence of the teeth, which moreover varies from one tooth to another, with values of 15.1° or 15.8° in FIG. 3 of this document of the prior art.
The patent EP0872943 discloses another example of an electrodynamic rotating machine with permanent magnets having a stator with a concentrated winding.
The patent application WO93/07672 is also known, which relates to a structure of motor with straight teeth. However, the proposed topology has 4 pairs of magnetic poles on the rotor, which does not allow a natural compensation of the sixth harmonic of the residual torque and requires modifications to be brought to the shape of the rotor magnets in order to reduce the residual torque. Besides, if this structure uses straight teeth, the latter have a large width (approximately 50% of the pole pitch), which is not optimized.
In a certain number of its patents, the applicant provided solutions making it possible to remedy all or part of the disadvantages mentioned above by using straight teeth topologies associated with rotors having 5 or 7 pairs of poles while showing that configurations could be found, in terms of width of stator teeth, for which the torque without current could be eliminated or at least significantly minimized. In particular, the U.S. Pat. No. 8,102,093 relates to a motor having 12 straight stator teeth, with 6 being wound, and 6 not being wound, with the particularity that the wound teeth are at least twice as wide as the non wound teeth. The wide teeth carrying the coils thus make it possible to maximize the Km value under certain conditions thanks to an increased permeance. The teeth width is so selected as to obtain an optimal compensation of the sixth harmonic of the residual torque appearing due to the not equal width between the teeth.
This type of structure perfectly meets the needs for a reduction in the torque without current, for balanced radial forces, enables an independent winding on coil bodies added on the stator subsequently and leads to satisfactory levels of performance. However, if constant axial overall dimensions and diameter are used, it surprisingly appears to the persons skilled in the art that, in some cases, this topology does not lead to the optimal result and that it would be profitable to have equal but narrower teeth than the teachings of the prior art would suggest in order to optimize the motor Km value. For example, even though the height of the stator has to be reduced, when using larger winding coil heads, to keep a constant overall dimension, better configurations, in terms of Km value, are obtained with 12 narrow teeth having an equal width, than with 12 teeth having different sizes, and this with teeth widths smaller than half the pole pitch.